1. Field of the Invention
The present invention relates to a resonator mounted in a radio communication apparatus, which comprises a dielectric substrate and a line of a predetermined length that is formed on the dielectric substrate.
2. Description of the Related Art
In the field of radio communication using high frequency, a necessary signal and an unnecessary signal are classified by taking out a signal of specific frequency out of many signals. A circuit performing this function is generally referred to as a filter and is mounted in a number of radio communication apparatuses. A resonator constituting a filter and having a line structure needs a line length of a quarter of or a half of the wavelength of the resonance frequency. In such resonators, a center frequency and a bandwidth which are design parameters are mostly fixed. When a plurality of frequency bands are used in a radio communication apparatus using these resonators, there is a method in which a plurality of resonators respectively having a different center frequency and a different band width are provided, and a resonator to be used is selected by switching a switch and the like.
It is also a considered method to combine a variable capacitive element with an inductance element having a line structure for obtaining a desired resonant frequency, instead of using the plurality of resonators. As an example of the method, contents described in paragraph 0004 and FIG. 2 of Japanese Patent Application Laid Open No. 6-61092 (hereinafter referred to as “document 1”) is shown in FIG. 1. An input strip line 273 provided with an input terminal 272 formed on an insulator 271 on a ground substrate 270 is connected to a movable electrode 277 formed on a displacement surface 276 of mechanical displacing means 275. The mechanical displacing means 275 is held by a structure body 278 for fixing it. Parts of the ground substrate 270 facing the movable electrode 277 projects from other parts, and an electrode 279 is formed on the surface of the projecting ground substrate 270, so that the movable electrode 277 and the electrodes 279 constitute a variable capacitive element. The movable electrode 277 is connected to a strip line 281 serving as an inductive reactance, which is formed on an insulator 280 on the ground substrate 270, and of which end is grounded. A gap d is changed by changing the position of the movable electrode 277, so as to make a capacitive reactance of the variable capacitive element form between the movable electrode 277 and the electrode 279 changed, as a result of which the resonance frequency is changed.
Besides the above described method, there is also an example described in the paragraph 0018, FIG. 2 of Japanese Patent Application Laid Open No. 7-321509 (hereinafter referred to as “document 2”). There is also proposed a method in which capacitors are arranged outside the resonator, instead of using the mechanical displacing means, and the resonance frequency is changed by selectively connecting the externally arranged capacitors.
In order to lower the resonance frequency of a resonator having a line structure, it is necessary to extend the line length. The line length needs to be doubled in order to halve the resonance frequency. Therefore, there is a problem that the resonator becomes large. For example, when the resonance frequency change from 4 GHz to 2 GHz, in the case of a quarter wavelength resonator, the line length needs to be doubled from 18.75 mm to 37.5 mm. This is an example in the case where the wavelength shortening effect of a dielectric substrate is not considered, but even when the effect is taken into consideration, the condition that the line length needs to be doubled in order to halve the resonance frequency, is not changed.
The conventional variable resonator of which resonance frequency can be changed, has also disadvantages that mass productivity is poor because the capacitive reactance component is changed by using the mechanical displacing means, and that reproducibility of the resonance frequency is low because the mechanical displacing means is liable to be affected by the ambient environment.
In the method in which capacitors are arranged outside a resonator having a line structure and selectively connected, small chip capacitors so-called 1005 having a width of 0.5 mm and a length of 1.0 mm are used as the capacitors. In the method, in addition to the size of the capacitor elements themselves, wirings for conducting signals are needed, as a result, the resonator becomes large. Further, the resonator has a common disadvantage that the resonance frequency is changed due to the deviation in mounting the chip capacitors and thereby reproducibility of the resonance frequency is poor.